Many industries that have recycled wastes to recover valuable components are now finding that their recovery processes present a risk of harm to the environment, in light of the present understanding of requirements for proper disposal of potentially toxic contaminants. Such processes may produce solid waste residues, once thought to be suitable for conventional landfill disposal, that must now be treated as hazardous waste. Such processes originally designed for efficient recovery of a valuable component, must now be redesigned to insure that waste residues remaining from the recovery contain no harmful constituents that must be disposed of into the environment.
In the aluminum smelting industry, fluorides and alumina have long been recovered from wastes generated in the electrolytic smelting of aluminum ores. The electrolytic process generates a number of waste materials that contain, for example, high percentages of fluorides and toxic quantities of cyanide that must be recycled into the process or otherwise disposed of into the environment. Recycling of such waste is sometimes difficult because contaminants may build up in the system over time and ultimately adversely affect product quality. An example of a waste that generally cannot be directly recycled to the smelter is spent carbon cathodes of the electrolytic process. The carbon cathodes are formed into a potliner for containing the aluminum-salt solution that is electrolyzed to produce aluminum. Over time the potliners become impregnated with high levels of fluorides, alumina and sodium. In addition, toxic components such as cyanides become a component of the liner. Other similarly contaminated aluminum smelter wastes are recovered from channel, trench and floor sweepings, spent alumina scrubber absorbents and the like.
The processes employed for treating aluminum industry wastes focus upon the efficient recovery of valuable hydrogen fluoride that is easily recycled into the smelting process. Each of the known processes produces a solid waste residue that includes a significant soluble fluoride residual and a relatively high level of other contaminants such that the residue is generally not acceptable for direct recycling into the aluminum reduction process. These residues are presently placed in a landfill or simply allowed to accumulate on the smelter site as a tailings pile or in sludge ponds. It is these residues which, when contacted with environmental fluids such as ground water, may contaminate water supplies and other valuable resources.
A number of the prior processes for handling aluminum electrolysis wastes involve, for example, pyrohydrolysis of the smelting wastes for recovery of the fluoride values. Such a process is described by Kruger et al., U.S. Pat. No. 4,362,701, in which waste products of carbon, Al.sub.2 O.sub.3, Na.sub.2 O and fluorine are pyrolized in a rotating kiln with steam and oxygen introduced countercurrently at temperatures on the order of 1200.degree.-1800.degree. C. Kruger recognizes that recovery is enhanced by high temperature, but that if temperatures are too high, there is formation of slag and encrustations that impede the continuity of the operation. Thus, Kruger insures that the feed material remains in particulate form by initially combining the feed with a substantially inert component that does not soften at process temperatures.
Bell et al. in U.S. Pat. No. 4,113,832 pyrohydrolyzes spent pot lining material by introducing the material into a furnace in contact with water at 1100.degree.-1350.degree. C. The process generates an off-gas which may be utilized in the production of NaF or AlF.sub.3. The process produces a solid clinker-type material that is subjected to alkaline digestion for recovery of an alkali aluminate solution from which high purity aluminum is recovered.
A number of the prior art processes pyrohydrolyze solid carbonaceous aluminum smelter wastes for fluoride recovery by contacting with a fluidizing stream of steam or air in a fluid bed furnace reactor. Such processes are described by Anderson et al. in U.S. Pat. Nos. 4,160,809 and 4,065,551.
None of the processes described in the literature produce a final waste residue from the recovery that is rendered inert to health and environmental risks. Thus in Anderson et al., in U.S. Pat. No. 4,158,701 and Bell et al. in U.S. Pat. No. 4,113,832 the clinker solid residual produced is extracted for alumina recovery, but results in a final residue containing all of the undesirable impurities which must be then disposed of into the environment.